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排序方式: 共有1051条查询结果,搜索用时 15 毫秒
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S. LIN H. G. BEOM D. TAO Y. H. KIM 《Fatigue & Fracture of Engineering Materials & Structures》2009,32(7):580-586
The dielectric breakdown of an unpoled piezoelectric ceramic, PZT807, with a conductive channel, is investigated. Cylindrical bar specimens with a conductive channel are used for breakdown tests of the unpoled piezoelectric ceramic under purely electrical loads. Narrow tubular channels emanating from the head of the initial channel are observed in the specimens after breakdown occurs. The radius of the tubular channel that is created at the surface of the initial channel head is insensitive to various types of channel formation. The problem of a fine tubular channel that emanates from the initial channel head is numerically solved to evaluate the three-dimensional J integral, which is directly related to the energy that is available at breakdown, at the initiation of a new channel in the specimen. The critical J integral at the onset of breakdown is obtained. 相似文献
995.
远程分布式光纤温度传感系统的设计和制造 总被引:6,自引:0,他引:6
为了测量长距离大范围空间分布温度场,设计制造出30km远程光纤温度传感器。基于光纤放大的反斯托克斯背向拉曼自发散射测温原理,对系统进行了优化设计。用1550nm掺铒光纤激光器作为抽运源,采用高速瞬态波形采样技术,累加平均等信号处理技术,解决了远处背向反斯托克斯弱信号的检测问题。采用了智能化恒温技术,使主要元器件在恒温条件下工作,解决了工程应用中环境的适应性。远程分布光纤拉曼温度传感器系统已达到的主要技术指标如下:光纤长度31km,测温范围0-100℃(可扩展),温度测量不确定度为±2℃,温度分辨力为0.1℃,测量时间为432s,空间分辨力为4m。 相似文献
996.
Crude rice bran oil 0, 1%, and 2% (w/w), was added to restructured beef roasts that were stored at 4C and analyzed at 0, 7, and 14 days to determine nutritional properties and oxidative stability. The saturated fatty acid to unsaturatedfatty acid (SFA/UFA) ratio and the content of 7‐ketocholesterol decreased (p < 0.05) whereas vitamin E vitamers increased (p < 0.05) in the product with 2% rice bran oil. TBARs numbers were lower (p < 0.05) in roasts with rice bran oil after 7 days of storage. The addition of 2% rice bran oil (w/w) was effective in improving both oxidative stability and vitamin E levels. 相似文献
997.
The demands for higher data rate in Wireless personal area network (WPAN) than one in Bluetooth has been completed with IEEE 802.15.3 standard. The standard, named High-rate WPAN (HR-WPAN), adopts TDMA-based Medium access control (MAC) protocol. In this paper, we propose novel MAC protocol and scheduling algorithm to achieve efficient time-slots allocation. Time slots for data transmissions are allocated by two steps. In the first step, the time slots are initially allocated using a proposed allocation algorithm based on statistical packet inter-arrival time. Then, the initial allocation is dynamically adjusted by utilizing feedback information coming from each device. Performance evaluations are carried out through extensive simulations and significant performance enhancements are observed. 相似文献
998.
In this paper, we investigate the issues of extending the Packet fair queueing (PFQ) algorithms into continuous channel model. We present a PFQ-based Opportunistic scheduler (PFQ-OS), which can improve user throughput while satisfying strict short-term fairness. We decouple the conflicting design objectives of fairness and user throughput improvement by introducing a Search radius (SR) into the framework of PFQ. We develop an analytical model and derive the fairness properties of PFQ-OS, which are related to the SR. Thereby system operators can focus on opportunistic scheduling for the users within the SR without violating the fairness guarantee. We further investigate the designing of scheduling policy under the strict fairness constraints, and propose that in order to utilize the opportunistic scheduling all the time, the selection rule should provide all the users with equal chance in being scheduled at each selection. Accordingly, we present Maximum relative SNR (Max-rSNR) selection rule for PFQ-OS. We derive the bounds in terms of user throughput under Rayleigh fading channel. An extensive set of simulations testifies our derived theoretical properties that PFQ-OS can significantly improve user throughput while providing excellent short-term fairness guarantee. 相似文献
999.
补料分批式青霉素发酵的机理模型已得到深入研究,但是模型往往难以用于补料的优化和批次内的控制。为了对模型进行优化控制,针对Birol等提出的青霉素发酵非结构动力学模型,合理调整了温度和pH变化的影响,得到了青霉素发酵过程的简化机理模型。反应基质的补料是青霉素优化控制的关键,选择对补料速率进行优化来提高青霉素的产量。由于机理模型具有非线性和约束条件,采用序贯二次规划算法来进行求解,其中将补料轨线进行分段处理提高了优化效率。优化计算结果表明改进的补料过程可以提高青霉素的浓度和产量。 相似文献
1000.
In this study, the structural control strategy utilizing a passive tuned mass damper (TMD) system as a seismic damping device is outlined, highlighting the parametric optimization approach for displacement and acceleration control. The theory of stationary random processes and complex frequency response functions are explained and adopted. For the vibration control of an undamped structure, the optimal parameters of a TMD, such as the optimal tuning frequency and optimal damping ratio, to stationary Gaussian white noise acceleration are investigated by using a parametric optimization procedure. For damped structures, a numerical searching technique is used to obtain the optimal parameters of the TMD, and then the explicit formulae for these optimal parameters are derived through a sequence of curve-fitting schemes. Using these specified optimal parameters, several different controlled responses are examined, and then the displacement and acceleration based control effectiveness indices of the TMD are examined from the view point of RMS values. From the viewpoint of the RMS values of displacement and acceleration, the optimal TMDs adopted in this study shows clear performance improvements for the simplified model examined, and this means that the effective optimization of the TMD has a good potential as a customized target response-based structural strategy. 相似文献